Method for making anhydrous chromium sesquioxide by reducing a hexavalent chromium compound

ABSTRACT

A METHOD OF MAKING ANHYDROUS CHROMIUM SESQUIOXIDE BY REDUCING A HEXAVALENT CHROMIUM COMPOUND NOT DECOMPOSABLE AT A TEMPERATURE OF LESS THAN 300*C., COMPRISING SUBJECTING SAID HEXAVALENT CHROMIUM COMPOUND TO HEATING IN A NON-REDUCING ATMOSPHERE BETWEEN ABOUT 197 AND 280*C., THEN HEATING IN A REDUCING ATMOSPHERE BETWEEN ABOUT 250 AND 350*C.

United States Patent 1m. (:1. 1344a 5712; C01g 37/02 US. Cl. 117-119.6 Claims ABSTRACT OF THE DISCLOSURE A method of making anhydrous chromium sesquioxide by reducing a hexavalent chromium compound not decomposable at a temperature of less than 300 C., comprising subjecting said hexavalent chromium compound to heating in a non-reducing atmosphere between about 197 and 280 C., then heating in a reducing atmosphere between about 250 and 350 C.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a method for making anhydrous chromium sesquioxide by reducing a hexavalent chromium compound which is not thermally decomposed at a temperature of less than 300 C., preferably chromic anhydride.

(2) Description of the prior art It is known that the dissociation of chromic anhydride, CrO into chromium sesquioxide, Cr O and oxygen is complete in an oxidizing atmosphere only at a temperature of more than 500 C. In certain cases it is, however, desirable to be able to reduce the chromic anhydride at a temperature of less than 500 C.

A first case here involves the production of chromium sesquioxide catalysts on a porous support, for example, a support made of alumina, on the basis of reducible hexavalent compounds of chromium. Supports, soaked in a solution of a hexavalent compound, for example, chromic anhydride, are heated up to the complete transformation of the latter into chromium sesquioxide. Now, the higher the temperature of the transformation of chromic anhydride into chromium sesquioxide, the smaller will be the specific surface of deposit and the weaker will be the activity of the catalyst.

Another case involves a treatment process for porous ceramic masses, which are partially fused at low temperatures or substrate coatings using successive cycles of chromic anhydride impregnation and the transformation of the latter (chromic anhydride) into chromium sesquioxide, in situ, with a view to the production of hard ceramic masses or hard coatings, by the methods set forth in French Pat. No. 1,580,247, dated May 3, 1968, and Belgian Pat. No. 726,180, of the Kaman Sciences Corporation. These treatments give porous masses and treated coatings the characteristics of mechanical resistance and surface hardness properties, resistance to abrasion and wear and tear, reduction of the coefiicient of friction and the like, which are particularly worthwhile and which are comparable, if not superior, to those which could be obtained by complete baking at high temperature. Furthermore, these treatments are administered without any appreciable delays; the objects to be treated can be machined and lined up side-by-side in the porous state, that is to say, in a reduced hardness state which facilitates the milling, thus not requiring any special and expensive tools, such as diamond milling tools.

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Although they are performed at a relatively low tem-- perature, from a practical standpoint on the order of 500 to 700 C., these treatments can, in certain cases involving narrow tolerances or particular configurations, bring about unacceptable deformations or differences in the sides, particularly in the case of coatings on metal substrates. In this case, likewise, the metal can undergo harmful changes in terms of structure, texture, or mechanical properties.

A reduction of the treatment temperature by several hundred degrees would thus be extremely worthwhile and would furthermore make it possible to impregnate porous products or to coat substrates, which could not stand up under treatment at 600 or 700 C. because of their low fusion or degradation temperature.

This would widen the choice of coatings that can be used with substrates to include products which could not be used, because of the difference of their expansion coefficient as compared to that of the substrate, at treatment temperatures of 500 to 700 C.

Tests involving the decrease in the transformation temperature used in the transformation of chromic anhydride into chromium sesquioxide, by using reducing agents, such as ammonium acetate, amine containing compounds, methyl amine, dimethyl amine, pyridine, were unsuccessful because of the explosive nature of the reactions.

It has also been proposed to prepare chromium sesquioxide by treating ammonium bichromate, first of all by heating in an atmosphere of a non-reducing gas, such as carbon dioxide or ammonia, between and 225 C., and then in a reducing atmosphere between C. and 250 C. Such a process, however, calls for considerable precautions to avoid the decomposition of the ammonium bichromate. In particular, the temperature, when heating under an inert gas, cannot be raised above 225 C. without risking the exposive decomposition of ammonium bichromate. But, at the level of the relatively low temperatures required here, the duration of this heating process must be rather long, on the order of 15 hours or as many as several days; this makes the process not at all worthwhile industrially.

An object of the invention is to remedy the inconveniences in the known methods. Its primary purpose is to reduce the temperature used in obtaining anhydrous chromium sesquioxide using a hexavalent compound of chromium, preferably chromic anhydride, CrO below 300 C., with sufliciently short reaction durations.

Another object is to transform the hexavalent compound of chromium into chromium sesquioxide without any explosive effect.

Another object of the invention is to apply this reaction in the production of catalysts involving a layer of chromium sesquioxide.

Another object of the invention is to apply this reaction to methods for making hard refractory ceramic substances, as described in French Pat. No. 1,580,247, and Belgian Pat. No. 726,180.

SUMMARY OF THE INVENTION The method involved in this invention is characterized by first of all subjecting the hexavalent compound of chromium to heating in a non-reducing atmosphere between 197 and 280 C., approximately, and secondly, to heating in a reducing atmosphere between about 250 and 350 C.

It furthermore preferably involves one of the following features:

A small quantity of a reduction catalyst, consisting of an amine containing organic compound, preferably pyridine, is added to the hexavalent compound of chromium.

3 7 Heating in non-reducing atmosphere is accomplished by raising the temperature to about 240 C.,' followed by maintaining this temperature for about 1 hour. The reducing atmosphere contains hydrogen. The reducing atmosphere is made up of a mixture of hydrogen and an inert gas, and the reduction takes place at a temperature of about 340 C. The reducing atmosphere is pure hydrogen.

The hydrogen is introduced progressively, first of all, in a mixture with an inert gas, and then as pure hydrogen. I Heating in the presence of hydrogen is accomplished by raising the temperature to about 280 C., after which this temperature is maintained for about 1 hour.

DETAILED DESCRIPTION OF THE INVENTION When it is desired to make a porous mass, impregnated with chromium sesquioxide, in accordance with the invention, we impregnate the porous mass with a solution of a hexavalent compound of chromium which is not decomposable at a temperature of less than 300 C., preferably chromic anhydride, and the latter is reduced by means ofthe methods involved in this invention.

' Different examples of employing the method according to the invention in the treatment of ceramic masses with a view to the production of hard ceramic masses, according to the methods involved in French Pat. No. 1,580,247 and Belgian Pat. No. 726,180, are now described below.

EXAMPLE 1 A small alumina stick, partially fused so as to present a porosity of 50% by volume, is subjected to impregnation in a solution of 1 kg./ liter of CrO The small rod is dried for 3 hours at 170-180 C. in a hot air oven, it is then placed in a sealed furnace whose temperature is raised up to 240 C., and it is then kept at this temperature for an hour. A current of hydrogen is then brought in at the same time that the temperature is increased to 280 C. small rod is pulverized and analyzed. The analysis reveals for an hour. After the treatment has been completed, the no trace of soluble CrO or hexavalent chromium salts.

A small rod treated under the same conditions, then brought to a temperature of 650 C. for 1 hour, revealed a weight loss of less than 1% of the weight of the oxide contained in it. The transformation is thus practically complete and it can be said that the CrO was transformed into Cr O to the exclusion of any intermediate oxides.

EXAMPLE 2 A certain number of small rods identical to those in Example 1 are distributed in two portions: one portion is subjected to the treatment described in Example 1, repeated until no further weight loss is registered. The other portion is subjected to the same number of impregnations, followed by a baking treatment in air at 670 C. Analysis does not reveal any appreciable difference between the two portions. The surface hardness obtained on the rods treat-.

ed in either group is essentially equivalent. Examination using X-ray defraction shows that the crystalline structure is identical in both cases but the degree of crystallinity and the dimension of the crystals are smaller in the case of the low temperature treatment.

Treatment of bodies impregnated with CrO under a reducing atmosphere, requires some precautions to avoid reactions which are abrupt. It is recommended to introduce the reducing gas only progressively, in a mixture with an inert gas (such as nitrogen or argon). Certain nitrated catalysts (for example, pyridine) facilitate the reaction and can be introduced into the treatment solution; for example, g./ liter of pyridine can be added to the solution of 1 kg./ liter of CrO EXAMPLE 3 Rods identical to those in Examples 1 and 2 are impregnated with a solution of 1 kg./ liter of CrO to which at 240 C. for 1 hour. The furnace is then allowed to cool to about 180 C. The hydrogen is introduced, drop by drop, stopping the flow of nitrogen, and the temperature is increased to 280 C. This temperature is maintained for /2 hour. The hydrogen is scavenged to remove the residual nitrogen and then to the drop by drop introduction of the hydrogen is again begun, maintaining the temperature at 280 C. for /2 hour. After 16 impregnations, followed by 16 treatments, a hardness of 96 Rockwell 15 N or 85 Rockwell 45 N is obtained, and a bending-resistance of 3000 kg./cm. measured on. rods with a diameter of 5 mm.

Furthermore, the rather unexpected result that the formation of anhydrous chromium sesquioxide with diluted hydrogen was more complete than with pure hydrogen was found. This result can probably be explained by the fact that water is formed more slowly and has less of a tendency to become fixed on the chromium sesquioxide with the formation of a hydrated oxide that is diflicult to return to the anhydrous state.

Other things being equal, the reduction speed is much a slower, it decreases as the hydrogen content decreases;

in order to reduce the duration of treatment, the heating temperature in the reducing atmosphere is slightly raised and brought up to about 340 C.

trogen and 20% by volume of hydrogen, introduced at a flow rateiif 20 liters per hour (the volume of the furnace was 4 liters). The temperature was raised to 340 C. and

kept at that level for 2 hours.

After treatment, certain rods were subjected to chemical analysis which revealed no trace of chromic anhydride,

CrO

After re-baking at 670 C., We found a weight loss of 1.1% of the weight of the oxide contained in it. v

The remaining rods" underwent 13 treatment cycles identical to the treatment described above, and were then sub.'

jected to chemical analysis which did not reveal the pres ence of CrO After annealing at 670 C., a weight loss of 1.1% of the weight of the oxide content was found.

After this treatment, the rods revealed a mechanical resistance to bending which amounted to 3150 kg./cm.

EXAMPLE 5 The same rod samples as in Example 4 were'subjected to the same impregnation and drying treatment and to the first baking phase.

The second baking phase was performed in an atmosphere involving a mixture of 99% by volume of nitrogen and 1% by volume of hydrogen, at 340 C., with one .8 hour stage at this temperature. Y

After this treatment, chemical analysis of the rods did not reveal the presence of CrO After annealing at 670 C., a weight loss of less than 1% of the weight of the oxide content was found.

The following experiments point up the needfor heat treatment in two phases, which characterizes the invention, that is, a first phase involving heating ata temperature of more than 197 C. and less than about 280 C. in a neutral or oxidizing atmosphere, a secondphase involving heating in a-reducing atmosphere at 250 to 35.0"

C., in order to obtain 'a result comparable to the result derived from heat treatment at 670 C EXPERIMENT 1 Absence of treatment at 197-280 C. in oxidizing atmosphere After drying at 170-180 C. for 3 hours, rods impregnated as in Example 1 were subjected to heating under hydrogen for one hour at 270 to 290 C. A second lot of rods, impregnated and dried in the same manner, were subjected to heating under hydrogen for an hour at 220 to 250 C. After treatment, the rods were weighed before and after the passage of an hour in a furnace at 670 C. The loss of weight was found and this loss corresponded to 13% unreduced CrO for the first lot and 50% unreduced CrO for the second lot.

EXPERIMENT 2 Absence of the first treatment stage at 197-280 C., in an oxidizing atmosphere, with treatment under hydrogen at 280 C., with a catalyst As catalyst, the solution indicated in Example 3, containing the addition of g./liter to a solution of 1 kg./ liter 010 was used. After treatment identical to the one described in Experiment 1, the rods were weighed before and after they had spent an hour in a kiln at 670 C. Weight loss was found and it amounted to 10.6% of the unreduced CrO EXPERIMENT 3 Increase in temperature of first treatment phase After drying the rods, impregnated as in Experiment 1, at 178 to 180 C., these rods were heated in the first phase in air at 280 C. to 290 C. for one hour. In the second phase, these rods were heated for 1 hour at 280 to 290 C. under hydrogen. After this treatment, the rods were weighed before and after heating at 670 C. The weight loss was found and this loss amounted to 23.0% of the untransformed C-rO These tests show that the result is not attained even after treatment in two phases, if the temperatures indicated here are not actually used; it is to be understood that the temperatures indicated here correspond to industrial treatment possibilities and that one can certainly imagine that certain of these temperatures could be lowered, which of course would mean that the treatment would be prohibitively prolonged, and that this would not be advisable from an economic viewpoint.

The examples given here are non-restrictive. They in- Wolve cases of applications where the CrO to be transformed came from an impregnation solution that contained only Cr0 The invention also applies to cases where the impregnation solution involves compounds of hexavalent chromium which can be transformed 'by reduction into Cr O or into trivalent chromium compounds, as well as solutions involving, in addition to CrO or compounds of CrO also additions of bodies that do not have to react or to be transformed in the course of the reduction from CrO into Cr O This may be the case, for example, in making mixed catalysts using impregnated porous supports.

The invention can also be applied to the case where the impregnation solution comprises a solution of CrO and a solution or suspension of oxides such as Cr O ZnO, MgO, A1 0 CiO and the like.

What is claimed is:

1. A method of making anhydrous chromium sesquioxide by reducing a hexavalent chromium compound not decomposable into said anhydrous chromium sesquioxide at a temperature of less than 300 C., comprising subjecting said hexavalent chromium compound to heating at substantially atmospheric pressure, first in a dry non-reducing atmosphere between about 197 and 280 C., then in a reducing atmosphere between about 250 and 350 C.

2. The method according to claim 1, wherein said hexavalent chromium compound is chromic anhydride.

3. The method according to claim 1, wherein a catalytic amount of a reducing catalyst comprising an amine containing organic compound is added to said hexavalent chromium compound.

4. The method according to claim 3, wherein said amine containing organic compound is pyridine.

5. The method according to claim 1, wherein the heating in a dry non-reducing atmosphere is accomplished by raising the temperature to about 240 C., followed by maintaining this temperature for about 1 hour.

6. The method according to claim 1, wherein said reducing atmosphere contains hydrogen.

7. The method according to claim 6, wherein said reducing atmosphere comprises a mixture of hydrogen and an inert gas and said heating in said reducing atmosphere is at a temperature of about 340 C.

8. The method according to claim 6, wherein the reducing atmosphere is pure hydrogen and wherein the hydrogen is introduced, progressively, first as a mixture with an inert gas and then as pure hydrogen.

9. The method according to claim 6, wherein the reducing atmosphere is pure hydrogen and the heating is accomplished by raising the temperature to about 280 C. and then maintaining this temperature for about 1 hour.

10. The method of making a porous mass impregnated with chromium sesquioxide, comprising impregnating a porous mass with a solution of a hexavalent chromium compound non-decomposable into said anhydrous chromium sesquioxide at a temperature of less than 300 C., subjecting the porous mass to heating at substantially atmospheric pressure, first in a dry non-reducing atmosphere between about 197 C. and 280 C., then in a reducing atmosphere between about 250 and 350 C.

References Cited UNITED STATES PATENTS 1,723,556 8/1929 Meyer et a1 23l45 2,854,313 9/1958 Banner 23-145 X FOREIGN PATENTS 1,580,247 9/ 1969 France 23145 OTHER REFERENCES Sisler et al.: Addition Compounds of Chromic Anhydride with Some Heterocyclic Nitrogen Bases, J. Am. Chem. Soc. (1948), PP. 3827-3830.

OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner US. Cl. X.R. 

